Pipelines made of medium to high density polyethylene or similar thermoplastic material are commonly used for transporting fluids such as natural gas. In making a buried pipe installation heat can be applied directly to the pipe ends by a suitable heating rod, and the pipe ends then pressed together to fuse the melted thermoplastic material and form a butt joint. However, once the installation is completed, replacement of a damaged pipe section is more difficult since the replacement pipe must be butted up tight against the adjacent undamaged sections. There is insufficient clearance between the pipe ends to insert a heating rod, and normally it is not possible to move the pipe sections relative to the surrounding earth.
In such instances a thermoplastic coupling or welding collar has been employed which receives the pipe sections within its opposite ends. The collar incorporates a helical heating coil or welding sleeve formed by continuous wraps or turns of wire sheathed in thermoplastic material and arranged along the inner surface of the collar in regular, longitudinally spaced apart relation. Energization of the welding sleeve raises the temperature of the interface between the welding sleeve and the abutting pipe sections to a molten state and forms a fusion bond. However, such a fusion welded joint has a number of significant shortcomings.
More particularly, when the interface between the welding sleeve and the pipe sections is heated to a molten state, the helical turns of the welding sleeve tend to float in the molten material. If they are not stabilized, such as by compressing them between the collar and pipe sections, some turns drift closer together and heat concentrates in that area, with correspondingly lower levels of heat in other areas where the turns are spaced farther apart. The areas of higher temperature tend to occur at the butt joint interface, and the thermoplastic material in that area approaches a molten state much more quickly than the remainder of the interface. As heating is continued in order to raise the remainder of the interface to fusion temperature, the thermoplastic material at the butt joint loses its structural rigidity and the pipe ends sag radially inwardly and intrude into the pipe flow passage. As melted material flows into the sag area, the heating coil turns migrate to that area and even closer together, compounding the sagging problem and accelerating the flow of molten material toward the butt joint area. By the time the interface throughout the complete length of the collar has reached proper fusion temperature the sagging adjacent the butt joint has created "hot spots" where the pipe walls are burned and structurally weakened.
The thermal sagging of the pipe ends is worsened if the pipe ends are irregularly cut. Since dimensional discrepancies or gaps can be as much as one eighth of an inch, the thermoplastic material flowing into these voids can rob the parent pipe of enough material that the pipe walls adjacent the joint are structurally deficient.
Sagging of the fused plastic into the pipe interior at the pipe ends not only adversely affects fluid flow, but the presence of sagged material at that point means there is probably insufficient fused material present along the remainder of the interface to provide a continuous bond. Also, any discontinuity in the bond results in gas leakage.
Attempts in the prior art to prevent radial inward sagging of the plastic material at the pipe ends have not been completely successful. U.S. Pat. No. 4,718,698, issued Jan. 12, 1988, for "Apparatus & Method of Forming Fusion Welded Butt Joint Between Thermoplastic Pipe Sections", and assigned to the assignee of the present application, teaches the use of a pair of flanged inner sleeves made of thermoplastic material. These were fitted within the pipe sections before placement of the welding collar over the exterior of the pipe sections. The sleeve flanges were abutted so that their melting and fusing would fill any dimensional discrepancies between the abutting pipe ends. The remainder of the sleeves did not melt but instead provided support against radial inward thermal sagging of the pipe ends. This generally solved the problem of sagging adjacent the butt joint, but at the expense of unwanted complexity and cost.
Another attempt in the prior art to solve "hot spot" heating adjacent the butt joint involved placement of two separately energizable heating coils in the welding collar on opposite sides of the butt joint. This method tended to isolate the pipe ends from excessive heating, but it was a relatively expensive expedient involving a multiplicity of electrical terminals and associated wiring. Further, it did not solve the problems of dimensional discrepancies between the welding collar and the pipe sections. Such discrepancies made it difficult to develop the necessary bonding pressures to stabilize the heating coil turns against thermal drift, to form a fusion interface layer of adequate length and continuity, and to eliminate gas inclusions such as air bubbles.
Such dimensional discrepancies result in large part from the fact that prior art welding collars are typically molded, and relief of the internal molding stresses on cooling changes the dimensions of such a collar so that it is often out of round. The resulting absence of intimate contact with the exterior surfaces of the pipe sections reduced bonding pressures and allowed air to be drawn into the bond interface during fusion heating, which adversely affected the depth and continuity of the fusion layer. U.S. Pat. No. 3,506,519, issued Jan. 13, 1916 for "Method of Making Interlocked Welded Connections Between Thermoplastic Articles", attempted to solve the lack of proper bonding pressures by employing an external pressure strap encircling the weld area. It was tightened to apply external pressure to the joint. However, the strap was not completely effective in collapsing the inner, partially softened bond area of the collar because it had to act through the relatively unsoftened, structurally rigid outer portion of the collar. As a consequence, the strap did not adequately maintain the welding sleeve in intimate contact with the articles to be joined, and did not develop the pressure necessary to stabilize the heating coil turns against drifting in the molten thermoplastic material.
Accordingly, there has been a need for a means to form a thermally fused circumferentially continuous leak free joint between pipe sections made of thermoplastic material, while preserving the structural integrity of the pipe sections without affecting fluid flow through the pipes.